This invention relates to brazing methods and more particularly to methods for brazing the joints of grids of nuclear fuel assemblies.
In many designs of nuclear reactors, the reactor vessel has an inlet and outlet for circulation of a coolant in heat transfer relationship with a core contained therein that produces heat. The core comprises an array or arrays of fuel assemblies which contain fuel elements. The fuel element is generally a cylindrical metallic sheath sealed at both ends containing nuclear fuel. The nuclear fuel which may be, for example, ceramic fuel pellets of a uranium compound is stacked in the fuel elements. During reactor operation, the fissionable isotopes within the nuclear fuel pellets absorb neutrons and subsequently fission generating heat as is well known in the art. The reactor coolant absorbs the heat while circulating through the core thereby cooling the fuel elements of the core and heating the coolant. Of course, the heated coolant may then be conducted to a location remote from the reactor vessel to produce electricity in a conventional manner.
In addition to fuel elements, typical fuel assemblies comprise top and bottom nozzles located at the ends of the fuel assembly which serve to hold the fuel elements together in an array. The top and bottom nozzles also provide mechanisms by which the fuel assembly may be held upright within the reactor core and may be held during transit to and from the reactor core. The fuel assemblies may further comprise grids spaced at various locations along the length of the fuel elements between the top and bottom nozzles that hold the fuel elements in appropriate relationship to each other. The grids also serve to provide spaces between the fuel elements for accommodating control rods. The grids may comprise thin metal straps having slots therein for accommodating the insertion of complimentary similar straps. When the straps have been inserted into the slots, the set of straps form a rectangular grid of rectangular holes for accomodating fuel elements or control rods. This type of grid has a configuration that resembles an "egg crate" structure. With the fuel elements disposed in the rectangular holes the grids thereby serve to properly space and align the fuel elements. Furthermore, flow vanes may be disposed on the grids for enhancing heat transfer. Typical grid structures are described and shown in U.S. Pat. Nos. 3,379,617 and 3,379,619, both issued in the name of H. N. Andrews et al.
Once the straps have been interconnected by means of their respective slots, it then becomes necessary to permanently bond the straps to each other. One such method known in the art for bonding the straps together is to apply braze metal by means of an applicator air gun along the intersection of the straps. When the braze metal has been so applied, the grid may be placed in a furnace so that the braze metal may braze the straps together along the line defined by their intersection. While the use of an air gun applicator to apply the braze metal has been generally effective, there has been an established need to be able to apply an exact predeterminable amount of braze metal to the joint.